1. Field of the Invention
The present invention generally relates to image projection systems, and more particularly to a portable overhead projector which collapses into a flat state in a single step.
2. Description of the Prior Art
Overhead projectors are widely used for educational and training purposes in schools and businesses, particularly in presenting visual aids during lectures. Such projectors generally comprise a base, a light source (with associated control electronics), and a projector head positioned above the base, housing an optical lens and mirror system. In the simplest form, the base includes a transparent window or stage, and the light source, typically an incandescent lamp, is placed below the stage. In this manner, when a transparency containing a visual image is placed on the stage, light from the lamp passes through the transparency, is focused by the lens in the projector head, and then displayed on a screen or wall. This type of projector is appropriately referred to as a transmissive projector.
Some projectors are used only at a single location, but it is often necessary to carry a projector from site to site. Therefore, it is desirable to provide a portable projector which is relatively compact and conveniently transported. One manner in which a projector may be made more compact is by providing a reflective base, instead of a transmissive base. In this construction, the base is very thin and the stage resting on the base comprises a reflective surface, such as a Fresnel mirror. The light source is placed in the projector head, with a lens collecting and directing the light toward the stage area. When a transparency is placed on the stage, light from the projector head passes through the transparency, is reflected and condensed by the Fresnel mirror, and passes up through the transparency a second time, then being focused and displayed by the lens and mirror within the projector head. This type of projector is referred to as a reflective projector.
While reflective projectors reduce the depth of the base, these units are still difficult to transport due to the projector head extending above the base. Projector heads have accordingly been devised which collapse or fold toward the base, further reducing the profile of the projector for transportation and storage. The easiest way to achieve this is by pivotally connecting the support arm of the projector head to the base. The head may then be swung over, or next to, the base for portability. For example, in U.S. Pat. No. 4,449,800 issued to De Longis et al. on May 22, 1984, the support arm hinges at the edge of the base, and the projector head actually detaches from the support arm.
Several variations of this design have been constructed, one of which is illustrated in U.S. Pat. No. 4,824,239 issued to T. Sekine on Apr. 25, 1989. In that device, two support arms are fixed to an axle which is rotatably mounted to the base. The projector head is also pivotally mounted to the support arms. When collapsing the projector, the head is first swung down and tucked between the support arms. The axle is then rotated, placing the head flush with the base. The device includes mechanisms to lock the arms and head into place when the projector is set up for use. A similar designed is disclosed in U.S. Pat. No. 4,588,271 issued to B. Emura on May 13, 1986. The support arm may have two or more pivoting joints as described in U.S. Pat. Nos. 3,653,754 (issued to E. Yamanaka on Apr. 4, 1972) and 4,696,557 (issued to M. Tomizuka on Sept. 29, 1987).
Another technique for optimizing the shape of the collapsed projector involves the construction of the projector head itself. The height of a standard projector head is increased by the presence of the mirror which is used to redirect the visual image to a vertical surface, i.e., a screen. The mirror must be oriented at an inclination of approximately 45.degree. to achieve this effect. It has been found, however, that the mirror may be pivotally mounted to the head itself, and folded down when not in use. This construction is used in prior art devices, including that shown in U.S. Pat. No. 4,824,239, and greatly diminishes the height of the head.
One disadvantage in the use of the foregoing projectors involves the multiple steps that are necessary in setting up or collapsing the projector. Because the set up is often done in the presence of the audience, it is highly desirable that this procedure be as simple as possible, so as to reinforce the image of the operator as a competent and professional person. If the operator is fumbling with the projection device, this will undoubtedly detract from the presentation. If, on the other hand, the operator is quickly and adroitly able to set up the projection equipment, this will enhance the overall effectiveness of the presentation. This is especially important when the operator is not familiar with the projector.
One overhead projector, manufactured by Liesegang Co. of West Germany, improves upon the prior art in this regard. That device includes a support arm comprised of upper and lower sections which are coupled together by a knee joint having a pull ring. The lower section is connected to the base, and the upper section is connected to the projector head. When the ring on the knee is pulled, the upper and lower sections of the support arm fold together, bringing the head flush with the base, over the stage area. A gear system inside the sections maintains the projector head in a horizontal orientation throughout the movement. After the mirror is folded downward, the device may be placed in protective luggage for transportation. This design, therefore, simplifies set up and collapsibility.
The Liesegang overhead projector, however, still requires two steps in the set up of the device, since the mirror must be manually adjusted. In fact, a user of that device who is not familiar with the projector head optics might not even realize that the mirror must be raised when setting the projector up. Unfortunately, there are no prior art portable projectors that provide for automatic adjustment of a movable projector head mirror. It would, therefore, be desirable and advantageous to devise a portable overhead projector which may be completely set up, or collapsed, in a single step.